1. Field of the Invention
This invention relates to an optical pickup device and an optical disc drive, and more particularly to those using a laser array in which two light sources with center wavelengths of 650 nm and 780 nm are formed on a common semiconductor substrate for the purpose of simplifying data recording/reading system for both DVD (digital versatile disc) and CD (compact disc) or CD-R (compact disc-recordable).
2. Related Background Art
Today, DVD systems have been brought into practice and are being spread as optical disc systems that can record massive data more than seven times than that of CD or CD-R (hereinafter called xe2x80x9cCDsxe2x80x9d). On the other hand, CD systems have been widely spread as optical disc systems. Therefore, in order to promote diffusion of DVD systems, it is desirable to provide DVD systems with compatibility with CD systems so that they can reproduce data not only from DVDs but also from CDs.
For reading of data from CDs, semiconductor lasers (laser diodes: LDs) for the wavelength around 780 nm are used. DVD systems, however, use LDs for the center wavelength 650 nm to realize the recording density about seven times of CDs. On the other hand, since the recording medium of CDs (particularly CD-R) is a pigment system material, sufficient sensitivity is not expected with LDs with the center wavelength 650 nm, and for providing DVD system with compatibility with CD systems, they are required to have a two-light-source optical pickup device for two center wavelengths 650 nm and 780 nm.
FIG. 1 is an explanatory diagram schematically showing configuration of a conventional optical pickup device having two independent light sources.
The conventional pickup device with two independent light sources shown in FIG. 1 includes a first optical integrated unit 101 for detection by emitting light with the center wavelength 650 nm, a second optical integrated unit 102 for detection by emitting light with the center wavelength of 780 nm, a dichroic filter 103 that transmits light whose center wavelength is 650 nm and reflects light whose center wavelength is 780 nm, a collimator lens 104 that collimates transmitted beams which are beams from the first optical integrated unit 101 and the second optical integrated unit 102 into parallel beams, a folding mirror 105 that change the direction of beams from the parallel direction to the vertical direction relative to an optical disc, a wavelength-selective iris 106 for adjusting the numerical aperture (NA) in accordance with the wavelength of light, and an objective lens 107 focalizing beams with center wavelengths of 650 nm and 780 nm which have been aligned in parallel by the collimator lens 104 onto optical discs. The first optical integrated unit 101 as the light source of light whose center wavelength is 650 nm and the second optical integrated unit 102 as the light source of light whose center wavelength is 780 nm are provided independently from each other.
Beams having the center wavelength 650 nm from the first optical integrated unit 101 pass through the dichroic filter 103 while spreading the beam diameter, and they are collimated into parallel beams when they pass through the collimator lens 104.
Thereafter, they are reflected by the folding mirror 105 to the vertical direction relative to the DVD 109, introduced into the objective lens 107 under adjustment of the numerical aperture by the wavelength-selective iris 106, focused onto the DVD 109 by the objective lens 107,and reflected by the DVD 109. Reflected beams from the DVD 109 contain data about the presence or absence of any record pits on the DVD 109, then return along the path of the emitted light in the opposite direction, and are detected by the first optical integrated unit 101.
On the other hand, the beams from the second optical integrated unit 102 having the center wavelength 780 nm are reflected by the dichroic filter 103 while spreading outward to impinge on the collimator lens 104, and are aligned into parallel beams when they pass through the collimator lens 104. Then, they are reflected into the vertical direction relative to the CD 108 by the folding mirror 105, then introduced into the objective lens 107 after being adjusted in numerical aperture by the wavelength-selective iris 106, focused onto the CD 108 by the objective lens 107 and reflected from the CD 108. Reflected beams from the CD 108 contain data about the presence or absence of any record pits on the CD 108, return along the path of the emitted light in the opposite direction, and are detected by the second optical integrated unit 102.
Since the CD 108 and the DVD 109 are different in spot size by the objective lens 107, the effective numerical aperture is usually adjusted by the wavelength-selective iris 106, for example, in accordance with the wavelength of light.
However, the conventional optical pickup device having two independent light sources requires complicated positional adjustment of two light sources to align their optical axes, and the use of two independent light sources makes it difficult to decrease the size of the device.
For the purpose of overcoming these two problems in the optical pickup device having two light sources, a double-source built-in semiconductor laser array having two light sources for center wavelengths of 650 nm and 780 nm on a common semiconductor substrate was developed to simplify the optical system (Japanese Patent Application No. hei 10-181068).
FIG. 2 is a cross-sectional view showing configuration of the double-source built-in semiconductor laser array.
The semiconductor laser array shown in FIG. 2 includes double heterostructures having different parameters, which are formed on different locations of a common semiconductor substrate, by substantially commonly designing upper parts of cladding layers of the double heterostructures in respective regions to integrate resonant elements which generate the light with the center wavelength 650 nm and the light with the center wavelength of 780 nm, respectively. Thus, this semiconductor laser array includes a laser element portion 240 for the oscillation wavelength of 780 nm and a laser element portion 241 for the oscillation wavelength of 650 nm.
In the laser element portions 240 and 241, sequentially stacked on a common gallium-arsenic GaAs substrate 21 are: n-type (n-) GaAs buffer layer 211, 221; n-In0.5(Ga0.3Al0.7)0.5P first cladding layers 212, 222; In0.5(Ga0.5Al0.5)0.5P optical guide layer 213, 223; multi-quantum well (MQW) active layers 214, 224; In0.5(Ga0.5Al0.5)0.5P optical guide layers 215, 225; p-In0.5(Ga0.3Al0.7)0.5P second cladding layers 216, 226; p-In0.5Ga0.5P etching stop layers 217, 227; p-In0.5(Ga0.3Al0.7)0.5P third cladding layers 218, 228; p-In0.5Ga0.5P cap layers 219, 229; n-GaAs current blocking layer 231; and p-GaAs buried layer 232.
In the laser element portion 240 for the oscillation wavelength 780 nm, the active layer 214 has a MQW structure including Ga0.9Al0.1As well layers and Ga0.65Al0.35As barrier layers. In the laser element portion 241 for the oscillation wavelength 650 nm, the active layer 224 has a MQW structure including In0.5Ga0.5As well layers and In0.5(Ga0.5Al0.5)0.5P barrier layers.
In the structure of the semiconductor laser array configuration shown in FIG. 2, by combination of the third cladding layers 218, 228 having a convex stripe configuration and the GaAs current blocking layer 231, steps of refractive indices are formed in the horizontal direction, and both laser element portions 240, 241 form refractive index-guided lasers. The GaAs current blocking layer 231 also functions to confine the current within the ridge stripe portion in each laser element portion. These element portions 240, 241 are electrically isolated by a separation groove 236, and they are independently driven via electrodes 233, 234. A minus-side electrode 235 can be formed on the bottom surface of the substrate 210 to be commonly used by both element portions. The laser element portion 240 for the oscillation wavelength 780 nm is used with CDs whereas the laser element portion 241 for the oscillation wavelength 650 nm is used with DVDs.
FIG. 3A is an explanatory diagram schematically showing configuration of a conventional double-source optical pickup device using a double-source integrated unit having a double-source built-in laser array, and FIG. 3B is an explanatory diagram schematically showing configuration of the double-source optical integrated unit.
The conventional double-source optical pickup device shown in FIG. 3A includes a double-source optical integrated unit 110 for emitting light with the center wavelength 650 nm and that of 780 nm, a collimator lens 104 that collimates transmitted beams which are beams from the double-source optical integrated unit 110 into parallel beams, a folding mirror 105 that change the direction of beams from the parallel direction to the vertical direction relative to an optical disc, a wavelength-selective iris 106 for adjusting the numerical aperture in accordance with the wavelength of light, and an objective lens 107 focalizing beams with center wavelengths of 650 nm and 780 nm which have been aligned in parallel by the collimator lens 104 onto optical discs.
Beams having the center wavelength 650 nm or 780 nm from the double-source optical integrated unit 110 are collimated into parallel beams when they pass through the collimator lens 104. Thereafter, they are reflected by the folding mirror 105 to the vertical direction relative to the DVD 109 or the CD 108, introduced into the objective lens 107 under adjustment of the numerical aperture by the wavelength-selective iris 106, focused onto the DVD 109 or the CD 108 by the objective lens 107, and reflected by the DVD 109 or the CD 108. Reflected beams from the DVD 109 or the CD 108 contain data about the presence or absence of any record pits on the DVD 109 or the CD 108, then return along the path of the emitted light in the opposite direction, and are detected by the double-source optical integrated unit 110. The light with the center wavelength 780 nm is used for the CD 108 whereas the light with the center wavelength 650 nm is used for the DVD 109.
The double-source optical integrated unit 110 shown in FIG. 3B includes a double-source built-in semiconductor laser array 111 in which two light sources are built on a common semiconductor substrate, an optical device 112 that directly transmits emitted light from the semiconductor laser array 111 but diffracts reflected light from an optical disc, and a photodiode (PD) for detection of signals and errors.
The semiconductor laser array 111 emits two kinds of light having the center wavelengths of 650 nm and 780 nm. Although the emitted light from the semiconductor laser array 111 directly pass through the optical device 112, reflected light from DVD or CD is diffracted to the position of the detection PD 113, and detected by the detection PD 113. The optical device 112 may be a hologram element, for example. A hologram element used as the optical device 112 may be provided integrally with the optical integrated unit 110 as shown in FIG. 3B, or may be provided separately from the optical integrated unit 110. It may be located between the collimator lens 104 and the folding mirror 105, between the folding mirror 105 and the iris 106, or between the iris 106 and the objective lens 107, for example.
When the hologram element is provided integrally with the optical integrated unit 110 as shown in FIG. 3B, it directly transmits emitted light from the semiconductor laser array 111, but diffracts diffracted light of a predetermined order in the reflected light from DVD or CD onto the position of the detecting PD 113, and converges it with the aid of the collimator lens 104. Usable as the hologram element is a micro diffraction grating having a transfer function so designed that light entering into a predetermined position on its surface be diffracted to a predetermined position on the detecting PD 113, and its pitch may be inconstant.
The detecting PD 113 has a plurality of divisional photo-detecting regions, and can detect focusing errors and tracking errors. In the illustrated example, both of 780 nm laser light and 650 nm laser light are detected by the common detecting PD 113.
In the illustrated optical system, since two laser emission points of the semiconductor laser array 111 are very close with the distance from 5 xcexcm to 500 xcexcm, two optical axes approximately overlap, and they can be regarded as a single common optical axis. That is, although there are two optical axes in the optical system shown in FIG. 1, the optical system shown in FIG. 3 aligns them into one common optical axis, configuration of the optical system is much more simplified.
As reviewed above, development and employment of a double-source optical integrated unit having a double-source built-in semiconductor laser array has made it possible to construct CD-compatible DVD optical pickup device and optical disc drive which are small in size and easy to adjust the optical system.
However, DVD and CD are different in thickness of the disc substrate, there was the problem that conventional devices could not sufficiently compensate the disc tilt property, defocusing property, jittering property and tracking property deteriorated by wavefront aberration (including spherical aberration) caused by the difference in thickness of the disc substrate. Normally, aberration optimization is effected for DVD with a strict specification. Therefore, wavefront aberration of CD during reading sometimes amounts to 0.5xcex (xcex is the wavelength), and may largely surpass 0.07 xcex rms which is considered the normally acceptable limit.
FIGS. 4A and 4B are explanatory diagrams schematically showing configurations of converged light onto DVD (FIG. 4A) and CD (FIG. 4B).
As shown in FIG. 4A, in DVD optimized in aberration, the focal point of the laser light is focused onto a disc signal surface 114. However, as shown in FIG. 4B, in CD with a thicker disc substrate than that of DVD, a large aberration is produced, and the focal point of the laser light is not focused to a point on the disc signal surface 114.
FIG. 5 is a graph which shows a relation between thickness of the disc substrate and wavefront aberration of an objective lens having the numerical aperture of 0.6 so designed to have no aberration when the wavelength of light is 635 nm and thickness of the disc substrate is 0.6 mm.
In case of the graph shown in FIG. 5, in DVD whose disc substrate is 0.6 mm thick, wave aberration is zero. However, in CD whose disc substrate is 1.2 mm thick, wave aberration is as large as 0.6xcex (xcex is the wavelength).
Such a large wavefront aberration causes focusing errors and tracking errors, and deteriorates qualities of CD-compatible DVD optical pickup devices and optical disc driving devices.
Heretofore, there were the following two techniques for correcting the wave aberration.
FIG. 6 is an explanatory diagram schematically showing configuration of a double-source built-in optical pickup device having a first wavefront aberration correcting device for the CD wavelength.
The double-source optical pickup device shown in FIG. 6 includes a double-source built-in semiconductor laser 117 for emitting light with the center wavelength 650 nm and that of 780 nm, a half mirror 103 which transmit about a half of the incident light and reflecting the other half, a collimator lens 104 that collimates transmitted beams which are emitted light from the double-source built-in semiconductor laser 117 into parallel beams, a special objective lens 118 which focuses light with the center wavelength 780 nm aligned into parallel beams by the collimator lens 104 onto the CD 108 and focusing light with the center wavelength of 650 nm onto the DVD 109, and a signal/error detecting PD 113 for detecting light with the center wavelength 650 nm and light with the center wavelength 780 nm which-are reflected light from the CD 108 and the DVD 109. The special objective lens 118 has a special shape for converging light entering into a central portion onto the CD 108 and light entering into the peripheral portion onto the DVD 109. The beams entering into the special objective lens 118, both with the center wavelength 650 nm or with the center wavelength 780 nm, are parallel beams. Although not shown for simplicity, a folding mirror for reflecting beams from the parallel direction to the vertical direction relative to the optical disc is interposed between the collimator lens 104 and the special objective lens 118. That is, the direction of the transmitted light from the collimator lens 104 and the direction of the transmitted light from the special objective lens 118 are normal to each other.
Light with the center wavelength 650 nm emitted from the LD 117 are aligned into parallel beams as a result of reflection by the half mirror 103 and transmission through the collimator lens 104. Then, it is reflected to the vertical direction relative to the DVD 109 by the folding mirror, and enters into the special objective lens 118. Part of the light with the center wavelength 650 nm entering into the special objective lens 118, which enters into the peripheral portion of the special objective lens 118, is focused onto the DVD 109, and reflected thereby. Although the light focused onto the DVD 109 is about a half of the light with the center wavelength 650 nm entering into the special objective lens 118, it is sufficient for getting recorded data from the DVD 109. Reflected light reflected by the DVD 109 contains data about the presence or absence of recording pits on the DVD 109. It returns along the path of the emitted light in the opposite direction, and after passing through the half mirror 113, it is detected by the detecting PD.
On the other hand, light with the center wavelength 780 nm emitted from the LD 117 is aligned into parallel beams as a result of reflection by the half mirror 103 and transmission through the collimator lens 104. Then, it is reflected to the vertical direction relative to the CD 108 by the folding mirror, and enters into the special objective lens 118. Part of the light with the center wavelength 780 nm entering into the special objective lens 118, which enters into the central portion of the special objective lens 118, is focused onto the CD 108, and reflected thereby. Although the light focused onto the CD 108 is about a half of the light with the center wavelength 780 nm entering into the special objective lens 118, it is sufficient for getting recorded data from the CD 108. Reflected light reflected by the CD 108 contains data about the presence or absence of recording pits on the CD 108. It returns along the path of the emitted light in the opposite direction, and after passing through the half mirror 113, it is detected by the detecting PD.
As explained above, the first wavefront aberration correcting device minimizes wavefront aberration for both the DVD 109 and the CD 108 by using the special objective lens 118 whose central portion and peripheral portion are different in focal distance.
FIGS. 7A through 7C are explanatory diagrams schematically showing configuration of a second wavefront aberration correcting device for the CD wavelength.
The second wavefront aberration correcting device uses ordinary elements as respective components of the optical pickup device, such as the wavelength-selective iris 106 and the objective lens 107, but it is configured to spread out the incident light to the objective lens 107 only when it is the light with the center wavelength 780 nm. That is, since aberration of DVD is already optimized, as shown in FIG. 7A, by introducing the light with the center wavelength 650 nm is introduced as parallel into the objective lens 107, it can be focused onto the DVD 109.
On the other hand, if the light with the center wavelength 780 nm is introduced as parallel beams into the objective lens 107, larger wavefront aberration is produced due to the thickness of the disc substrate of the CD 108, and the focal point of the laser light is not focused to a point on the disc signal surface 114 as shown in FIG. 7B. Therefore, in case of the light with the center wavelength 780 nm, by introducing it as spread light into the objective lens 107, wavefront aberration can be minimized, and the focal point of the laser light can be focused onto the CD 108 as shown in FIG. 7C.
However, in case of the first wavefront aberration correcting device, it is necessary to shape the objective lens into a complicated, special form, and there are a lot of difficulties for actual mass production and practical use, when selection of materials, preparation of an accurate mass-production mold, manufacturing cost for mass-production, and so on, are taken into account.
Additionally, regarding the second wavefront aberration correcting device, it is easy to bring it into practical use when using two independent light sources. However, when a double-source built-in semiconductor laser array including two light sources formed on a common semiconductor substrate is used, since positions of two light sources in the optical axis direction overlap with each other, light only from one of the light sources cannot be introduced as spread light into the objective lens by using ordinary elements as respective components of the optical pickup device.
It is therefore an object of the invention to provide an optical pickup device and an optical disc device having two built-in light sources, which include a wavefront aberration correcting device having a relatively simple structure and capable of minimizing wavefront aberration during reproduction of data not only for DVD but also for CD.
According to the invention, there is provided the optical pickup device including a double-source built-in semiconductor laser for emitting light of a first wavelength and light of a second wavelength, a first divergence modifying device for modifying the diverging rate of emitted light from the double-source built-in semiconductor laser to a first diverging rate, a second divergence modifying device for modifying the diverging rate of a part of transmitted light through the first divergence modifying device to a second diverging rate, and an objective lens which focuses the light with the first wavelength modified to the first diverging rate by the first divergence modifying device onto a first optical disc, and focuses the light with the second wavelength modified to the second diverging rate by the first and second divergence modifying devices onto a second optical disc. Thereby, the optical pickup device according to the invention can minimize wavefront aberrations of first and second optical discs which generate different wavefront aberration due to a difference in thickness between their disc substrates, and can focalize focal points of laser light on disc signal surfaces of respective optical discs.
In the typical configuration, the first divergence modifying device may be a converging device for converging emitted light from the double-source built-in semiconductor laser into parallel beams, the second divergence modifying device may be a spreading device for changing the part of the transmitted light through the converging device into spread beams, the objective lens focalizing the light of the first wavelength modified into parallel beams by the converging device onto the first optical disc and focalizing the light of the second wavelength modified into spread beams by the converging device and the spreading device onto the second optical disc.
According to the invention, there is provided the whole configuration of the optical pickup device including a double-source built-in semiconductor laser for emitting light of a first wavelength and light of a second wavelength, a first divergence modifying device for modifying the diverging rate of emitted light from the double-source built-in semiconductor laser to a first diverging rate, a second divergence modifying device for modifying the diverging rate of a part of transmitted light through the first divergence modifying device to a second diverging rate, an objective lens which focuses the light with the first wavelength modified to the first diverging rate by the first divergence modifying device onto a first optical disc, and focuses the light with the second wavelength modified to the second diverging rate by the first and second divergence modifying devices onto a second optical disc, a reflected light separating device which separates reflected beams from the first and second optical discs away from the path of the emitted light from the double-source built-in semiconductor laser, and a detecting device which detects reflected beams from the first and second optical discs separated by the reflected light separating device.
According to the invention, there is provided the typical whole configuration of the optical pickup device including a double-source built-in semiconductor laser for emitting light of a first wavelength and light of a second wavelength, a converging device for converging emitted light from the double-source built-in semiconductor laser into parallel beams, a spreading device for changing the part of the transmitted light through the converging device into spread beams, an objective lens which focuses focusing the light of the first wavelength modified into parallel beams by the converging device onto the first optical disc and focuses the light of the second wavelength modified into spread beams by the converging device and the spreading device onto the second optical disc, a reflected light separating device which separates reflected beams from the first and second optical discs away from the path of the emitted light from the double-source built-in semiconductor laser, and a detecting device which detects reflected beams from the first and second optical discs separated by the reflected light separating device.
With the configuration of the optical pickup device according to the invention, it is possible to provide an optical pickup device having two built-in light sources, which includes a wavefront aberration correcting device having a relatively simple structure and capable of minimizing wavefront aberration during reproduction of data not only for DVD but also for CD.
More concrete configuration of the optical pickup device according to the invention will be explained later.
According to the invention, there is provided the optical disc device including the optical pickup device according to the invention, and an optical disc device for rotatory driving the optical disc.